P0171

The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM to control a 14.7 to 1 air/fuel ratio under normal driving conditions. The PCM can make fuel corrections from a nominal 128 (0% correction) short/long term fuel trim value to a maximum window of 70 to 165 (-46% to 32%). If the PCM determines a rich condition exists (02 voltage above 450 mV), it will decrease injector pulse width (reducing fuel) to maintain a 14.7 to 1 air/fuel ratio. The short term fuel trim value will be between 70 (-46%) and 128 (0%) indicating a rich condition. If the PCM determines a lean condition exists (02 voltage below 450 mV), it will increase injector pulse width (adding fuel) maintaining a correct air/fuel mixture. The short term fuel trim value will be between 128 (0%) and 165 (32%) indicating a lean condition. When certain conditions have been met to begin learning fuel control (ECT closed loop, etc.) the PCM will use the short term fuel trim cell to update one of the long term cells being used.

DTC PARAMETERS
DTC P0171 will set if the fuel trim index is above 165 (32%) for four seconds and:
^ Engine coolant is between 60° and 115°C (140° and 239°F).
^ Manifold pressure is above 26 kPa (4 psi) (manual) or 28 kPa (4 psi) (auto)
^ RPM is between 550-3600 (auto) or 750-3600 (manual).
^ Barometric pressure is greater than 72 kPa (10 psi).
^ IAT reading is between 25° and 115°C (-13°F and 239°F).
^ Fuel level is greater than 9.8%.
^ Vehicle speed is less than 132 km/h (82 mph).
^ No Engine Sensor, VSS, EVAP Canister Purge, Front O2S, Idle Speed, PCM Internal, Misfire, Fuel Injector or P0125 DTCs have been set.

IMPORTANT: The fuel control system is continuously checked when the above conditions have been met.

Possible Causes
The following are malfunctions that could cause a lean condition:
^ Fuel pressure too low. Perform fuel pressure test.
^ Vacuum leak.
^ Water in the fuel.
^ Exhaust leakage around manifold or front O2S sensor.
^ Front O2S signal circuit 412 shorted to ground. Check by reading O2S1 with ignition On, engine Off. If reading is less than 100 mV check circuit 412 for a short to ground.
^ Injector plugging.

IMPORTANT: If the O2S sensor 1 reading voltage is below 300 mV and the HO2S sensor 2 is below 300 mV all the time, the O2 sensors are probably not at fault.